Charger STV Performance
I own and operate a Canadian Built 1988 (20' 6") Charger STV Modified
Tunnel Hull boat. It is equipped with a CMC Jack Plate. I have recently
purchsed a new 2002 Mercury 2.5 L Sport High performance racing
application engine. Max RPM 7800. The 2.5 replaced a 200 HP 1988 Mercury
engine, equipped with a 28 Pitch Chopper. Mercury suggested a 26 pitch
Chopper on the new engine, the problem is that to this point, the old
engine has outperformed the new high performance engine.
Old 5400 rpm, 78mph. New 73 mph, max achievable rpm 6800. Max speed
generated at 6200 rpm. the engine/prop combination appears to provide
ample torque and bow uplift, but disappointing high performance. Any
advice would be greatly appreciated.
Don't feel bad as the same thing has happened to me many times. The 28
prop is always going to be more efficient than the 26 or the 24 you need.
It takes a lot of horsepower to overcome the lost efficiency. So there is
some bad news. I suspect that the new motor has a sportmaster and the old
one had a nosecone gearcase. There is some more MPH. It sounds like your
boat is hyper sensitive to little changes. Here's the easiest fix. Get a 2
to1 ratio nosecone 200 gearcase and run your old prop. The rpm would be
around 7000 and probably faster than the old 200. You could try 2 to 1
gears in the sportmaster but I know it will not match the 200 coned case.
You could find some miracle 25 or 26 prop but I think it will be a hard
task. You could try some different props and heights and let me know the
feedback. I think your old combo was very good and to go back to the
gearcase and prop would be the most accurate. We need the gear change to
get the rpm up.
Try some things and let me know. If little improvements start happening I
can help you go find the rest of the speed.
whatever questions you have to IHRTechTalk@yahoo.com.
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members. Randy can not be held responsible for any advice
given. Though his information and expertise is second to none, he
has no control over what you do with your boat. It is up to you to
boat safely and act responsibly, and his advice is only to be used as a
guidance for your high performance boat/motor of which you are the one
responsible for the risks involved.